As indicated in previous answers, some silent letters (such as silent final <e>) serve definite diacritic purposes. But in most cases silent letters simply indicate the way things used to be before certain consonant and vowel clusters simplified to make them easier to pronounce. In such cases a silent letter can help show to what other words the word with the silent letter is related. For instance, the silent <b> in debt shows its relationship to its source, the earlier Latin word debitum, and the silent <g> in sign shows its relationship to related words like signal and designate.
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